Steam promotes the transformation between crystalline phases, a suitable H 2 O/O 2 ratio is critical to maintain the cycle between the V5+-O-P O site and the V4+-O P OH site, which not only increases the quantity of active sites, but also preserves the stability of the VPO catalyst. [Display omitted] • The Lewis acid site V5+-O-P O is the active site for catalytic conversion of n -butane. • Steam-air-nitrogen atmosphere plays critical role during VPO activation process. • A high conversion of 89 % and MA selectivity of 72% was achieved over SZ-27%. Vanadium phosphorus oxide (VPO) catalyst activation method is essential for the catalytic oxidation form n -butane to maleic anhydride. Here, we evaluate the effect of steam content on VPO catalyst during the activation process and validate the adjustment of the steam content could modulate the VPO activity. The DRIFTS results reveal that the steam content governs the formation of P-OH and V-OH species on the surface of VPO catalyst, whereas these sites would not enhance the catalytic activity directly. In contrast, the L acid site V5+-O-P O, forming a mutual transformation relationship with the P-OH species under steam/O 2 conditions, is the main active site that dominates the conversion of n -butane, and a moderate amount of steam plays a role in stabilizing the V5+-O-P O site. [ABSTRACT FROM AUTHOR]